The invention relates generally to peck feed drilling equipment and more specifically to an automatic drilling machine which peck drills an opening by repeatedly advancing a drill bit into the work, drilling a short distance, fully retracting the drill bit, repeating this cycle until drilling is completed and then shutting off.
Accompanying the development of various machines and apparatus for drilling holes was a similar effort directed to associated equipment for providing and controlling drilling programs to ensure that the drilling operation was accomplished most expeditiously. Typically, such programs sought to provide the best and most uniform aperture with the least drill bit wear. For example, U.S. Pat. No. 2,657,595 teaches a drilling tool which utilizes a pressurized fluid controlled feed mechanism having a rapid approach, a controlled feed movement of a predetermined length and a rapid return of the tool to it starting position. U.S. Pat. No. 3,351,047 illustrates an apparatus in which a rotating cam having a follower coupled through cables provides a repetitively applied controlled force to a rotating tool for drilling, polishing and the like.
One approach to improving drilled aperture quality is to remove the feed rate variable from operator control and accomplish it automatically through the agency of the drilling mechanism. In U.S. Pat. No. 3,512,433 a positive feed drill is disclosed wherein an air motor drives both the drill spindle and the axial feed mechanism. Accordingly, a fixed nominal drive ratio exists between the rotation of the drill and its axial translation. Fluid operated brakes and clutches control the direction and axial speed of the drill. U.S. Pat. No. 3,663,138 also discloses a drilling mechanism wherein the drill bit is rapidly advanced to proximity with the workpiece and then slowly fed into the workpiece. Here, the axial motion of the drill is accomplished by a pair of interconnected, double-acting pistons.
A hole drilling program which provides excellent repeatability, concentricity and overall accuracy is the peck feed drilling method. Peck feed refers to a drilling program wherein a drill bit engages a workpiece and drills a short increment. The drill bit is then fully retracted and subsequently advanced into the workpiece to repeat the cycle and drill another short increment. Openings drilled by this advance, drill, retract, repeat cycle exhibit improved accuracy and concentricity, primarily because distortion of the workpiece caused by elevated workpiece temperature and chip machining of the aperture are both reduced. Accordingly, there are several machines and patents directed thereto which accomplish automatic or semi-automatic peck feed drilling.
For example, U.S. Pat. No. 3,704,957 discloses a mechanically operated and controlled peck feed assembly utilizing a cam and crank arm which axially reciprocates a drive motor and drill bit. A compression spring limits the axial force which can be applied to the drill bit.
Another peck feed drilling apparatus is illustrated in U.S. Pat. No. 4,350,212. This device includes a motor which operates a quill and drill bit through a speed reduction device and splined interconnection. The peck feed components include a hydraulic feed control device and a ratchet mechanism which cooperates with a feed control device to provide peck feed operation.
U.S. Pat. No. 4,362,444 presents another peck feed drill assembly having a cam operated advance and retract mechanism. The drill assembly includes a first motor which provides rotating power to the drill bit and a second which axially positions the drill bit. Accordingly, independent drill speeds and feed rates are possible.
A review of the foregoing prior art patents reveals that the basic concept of peck feed drilling and equipment for accomplishing such drilling are old. However, there are several aspects and problems of peck feed drilling which are neither acknowledged nor addressed by the prior art. For example, breakthrough of a drill upon hole completion is a critical operation because at a normal drilling force, the drill bit may break and tear through a small remaining portion of a work piece. One end of the drilled bore may thus be rough, incorrectly sized or non-circular. When such breakthrough occurs just as the drill bit commences a peck feed drill cycle, the breakthrough can be especially rapid, uncontrolled and damaging. The motor may also stall, necessitating backing the drill bit out of the hole and possibly restarting the drilling cycle. One approach to reducing such breakthrough damage is to operate the drill at a reduced feed rate throughout the drilling cycle. However, this is inefficient as the drilling operation will then take substantially longer than it would have if the drill were operated at an optimum feed rate throughout the drilling cycle.
Another difficulty unique to peck feed drilling assemblies is the impact of the drill bit against the bottom of the drilled hole in the work piece at the commencement of each drilling cycle. Once again, one solution to this problem has been to slow translation of the drill bit throughout the entire advance cycle. Unfortunately, this solution has the same consequence as above which is to slow the entire drilling process and thus render the overall drilling process more costly and less efficient.
A third problem inherent in some prior art drilling machines relates to the manner of controlling the peck drilling distance. In units which peck drill for a selected time interval, the distance drilled will be a function of the applied force, the drill speed, the material hardness, the hole depth and the drill sharpness. Given the variability of these parameters, especially the last one, it is clear that the drilled distance may vary. A control system which integrates force over time to compute peck distance fares little better, again because of the variability of drill sharpness and other factors.
From the foregoing, it is apparent that improvements in the art of peck feed drilling machines are both possible and desirable.